Molded electronic component having pre-plated lead terminals and manufacturing process thereof

ABSTRACT

A molded elecronic component such as a solid electrolyte capacitor comprising a capacitor element including a cathode layer, an anode lead, a pre-plated anode lead terminal connected to the anode lead, a pre-plated cathode lead terminal connected to the cathode layer, and an insulating member which encapsulates the capacitor element and leaves a portion of the anode and cathode lead terminals exposed. The encapsulated portions of the pre-plated anode and cathode lead terminals have a plating layer formed thereon containing organic substances in an amount of 0.03 wt. % or less. Also disclosed is a process for manufacturing the solid electrolyte capacitor.

This is a Divisional of application Ser. No.08/9,01,239 filed Jul. 25,1997, now U.S. Pat. No. 6,127,205 the disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a resin-molded electronic componenthaving lead terminals, and more particularly, to a chip type solidelectrolyte capacitor having solder plated lead terminals, and a processfor manufacturing the same.

2. Description of the Related Art

As the weight and size of electronic equipment is progressively reduced,and as surface mounting technology has advanced, the chip type componentsuch as a solid electrolyte capacitor featuring a small size and highcapacity has enjoyed an increasingly expanded market scale and is usedin various applications.

Conventionally, as shown in FIG. 8, a chip type solid electrolytecapacitor has a capacitor element 1 having a cathode layer 3, an anodelead 2, an anode lead terminal 8 connected to the anode lead 2 bywelding or the like, and a cathode lead terminal 9 connected to thecathode layer 3.

Moreover, the cathode layer 3 and the cathode lead terminal 9 areconnected by a conductive adhesive 10 such as silver paste. Thecapacitor is packaged by resin 6, leaving portions of the anode andcathode lead terminals 8 and 9 exposed. In addition, the anode andcathode lead terminals 8 and 9 are bent along the outer surface of theresin package 6. Since these lead terminals 8 and 9 are pre-plated witha plating layer of solder or the like by a direct current plating methodusing a linear current waveform as shown in FIG. 9, it has the followingdisadvantages (1) and (2).

(1) Organic components contained in a plating solution such as abrightener or dispersant are incorporated into the plating layer. As aresult, the plating layer of solder or the like formed on the anode andcathode lead terminals 8 and 9 contains a considerable amount of thefollowing organic substances.

Sample: Plating film formed by direct current plating

Detected components: Dichlorotoluene,

N,N,2,6-tetramethylbenzeneamine,

trichlorobenzene,

chlorobenzenemethanol,

dichlorobenzaldehyde,

dichlorobenzenemethanol,

lauric acid methyl

When such pre-plated lead terminals are employed in a resin-molded typeelectronic component, the organic substances in the plating layer ofsolder or the like are vaporized by thermal stress when mounting thecomponent to generate a considerable amount of gas.

The gas thus generated in an adhering interface between the conductiveadhesive 10 and the cathode lead terminal 9 affects connectionreliability, and reduces the connection strength between the conductiveadhesive 10 and the cathode lead terminal 9. In a worst case, thecathode lead terminal may peel off to result in an electrically opencircuit.

(2) The plated solder on the lead terminals 8 and 9 in the resin package6 tends to elute from the resin package to form a solder ball 7. Thisoccurs when an excessive amount of an organic substance is generated dueto thermal stress in mounting the electronic component. The resultingsolder ball 7 can short circuit conductive patterns when the componentis mounted on a printed circuit board.

To avoid the foregoing disadvantages, it is possible to plate only theexposed portion of the lead terminals. But such selective platingincreases production cost.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide anelectronic component such as a solid electrolyte capacitor whichmaintains good connection reliability at its connecting portion usingconductive adhesive and prevents the generation of a solder ball, and aprocess for manufacturing the same without resorting to a selectiveplating method.

The above objective of the present invention is achieved by emptyingelectroplated lead terminals which contain organic substances in anamount of 0.03 wt % or less. In the case of a solid electrolytecapacitor, a pre-plated anode lead terminal is welded to an anode leadof a capacitor element and pre-plated cathode lead terminal is connectedto a cathode layer of the capacitor element by using a conductiveadhesive. The capacitor element is encapsulated by an insulating memberso as to leave a portion of anode and cathode lead terminals exposed.Both of the encapsulated and exposed portions of the pre-plated anodeand cathode lead terminals have a pre-plated layer containing organicsubstances in an amount of 0.03 wt. % or less.

Such a small amount of the organic substances in the electroplated loyercan be achieved by using a special pulse plating method with a currentwaveform having a counter pulse. ON time and OFF time of the currentwaveform is selected to be about 10 to 500 msec and the OFF timeincludes a counter (negative) current pulse which has an amplitude thatis from ½ to {fraction (1/50)} that of the ON time pulse and a durationof 2 to 20 msec.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side cross sectional view of a chip type solid electrolytecapacitor according to an embodiment of the present invention.

FIG. 2A is a perspective view of a chip type solid electrolyte capacitorassembled in a lead frame prior to a molding prosess.

FIG. 2B is an enlarged perspective view showing a main part B shown inFIG. 2A.

FIG. 3 is a waveform diagram showing an example of the current waveformused in the pulse plating method of the present invention.

FIG. 4 is a waveform diagram showing an example of the current waveformused in a preferred embodiment of the present invention.

FIG. 5 is a diagram of a plating apparatus for use in the presentinvention.

FIG. 6 is a graph correlating impurity content of the plating layer andcathode connection strength.

FIG. 7 is a graph showing the rate of solder ball generation as afunction of impurity content.

FIG. 8 is a side cross sectional view of an example of a conventionalchip type solid electrolyte capacitor.

FIG. 9 is a waveform diagram showing an example of the current waveformused in a conventional direct current plating method.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, an anode body of a valve metal such as tantalum issubjected to anodic oxidation and a solid electrolyte later such asmanganese oxide layer, a carbon layer and a silver paste layer areformed thereon sequentially. As a result, a solid electrolyte capacitorelement 1 having a cathode layer 3 in the outer-most layer is formed. Asshown in FIG. 2A, anode lead 2 embedded in this capacitor element 1 anda pre-plated anode lead terminal 8 are connected together by means ofwelding or the like. A pre-plated cathode lead terminal 9 is connectedto the cathode layer 3 via a conductive adhesive 10. The assembly isthereafter encapsulated with an electrically insulating member 6 such asepoxy resin by means of a transfer molding process such that a portionof each of the anode and cathode lead terminals 8 and 9 is exposed.

Since a lead frame 900 is punched out from a solder-plated metal tape(not shown), lead terminals 8 and 9 are provided with a top solderplating layer 91 and bottom solder plating layer 92 and side edgesthereof are-not plated as shown in FIG. 2B. Needless to say, solderplating may be carried out for the lead frame after punching out from anon-plated metal tape. In that case, whole surfaces of the lead frameare coated with a plating layer.

The solder plating layer of the present invention is formed using apulse plating method with a current waveform having an ON time of fromabout 10 to about 500 msec including a positive current pulse and an OFFtime of from about 10 msec to about 500 msec including a negativecurrent pulse as shown in FIG. 3. The negative current pulse (counterpulse) has an amplitude that is from ½-{fraction (1/50)} that of thepositive current pulse and a duration of from 2 to 20 msec. Other thanthe counter pulse, preferably, there is no current flow during the OFFportion of the waveform. The timing of the negative current pulse is notrestricted to the position shown in FIG. 3, but may be at any positionbetween adjacent positive current pulses.

In a preferred embodiment, the current waveform has an ON time (positivepulse) of 200 msec and an OFF time of 200 msec including a 10 msecnegative pulse at the trailing edge of the positive pulse as shown inFIG. 4. The amplitude of the negative pulse is selected to be {fraction(1/10)} of the positive pulse. The pulse plating method of the presentinvention reduces the amount of organic substances (e.g., dispersantand/or brightener) from the solder plating layer having a thickness ofabout 4μm.

A well known plating apparatus can be used for this invention as shownin FIG. 5, including plating bath 13, electrode 14, lead terminal basemetal 15 and current source 16. When positive and negative pulses areapplied to the electrode 14, the base metal 15 may be grounded. Theelectrode 14 is made of Pb/Sn alloy and the solder plating bath contains10 to 60 vol. % of Sn(BF₄)₂, 1 to 40 vol. % of Pb(BF₄)₂, 10 to 70 vol. %of HBF₄ and 1 to 20 vol. % of additives. The base metal 15 is made ofwell known 42-alloy which is widely used for lead terminals ofelectronic components. Needless to say, the solder plating bath is notrestricted to the above described solution, for example, the solderplating solution disclosed in U.S. Pat. No. 4,589,962 may be used.

The connection strength between the cathode layer 3 and the cathode leadterminal 9 of the chip type solid electrolyte capacitor thus preparedprior to forming the external package was evaluated. The presentinventors found, as shown in FIG. 6, that the connection strength in thepresent invention was about 1.5 times that of a conventional device. Thefluctuation in connection strength was also reduced by 20% or more.

In addition, the product was conveyed into a heat furnace after theexternal package was formed to apply thermal stress thereon andinspected for solder ball generation. It was found that solder ballswere generated 75% of the time (75% solder ball generation rate) whenthe cathode lead terminal had a solder layer thereon formed using aconventional direct current plating method, while no solder balls weregenerated in making the solid electrolyte capacitor of the presentinvention.

Furthermore, as clearly seen from the relationship between the cathodeconnection strength and the impurity content (wt. %) as shown in FIG. 7,the present inventors found that solder balls substantially do not formwhen the organic substance (impurity) content in the solder platinglayer is 0.03 wt. % or less.

This effect is obtained by providing a plating layer on all of thesurfaces of the anode and cathode lead terminals or by providing onlyupper plating layer 91 of the anode and cathode lead terminals 8 and 9,which surface is subsequently soldered to a printed wiring board.

The content of organic substances (impurities) contained in a platinglayer formed on the anode and cathode lead terminals in a conventionaldirect current plating method is about 0.2 wt. %. On the other hand, inthe pulse plating method of the present invention, the impurity contentcan be reduced when the component is subjected to thermal mountingstress and a solder ball is not formed.

Although a simple pulse plating mehod is known to provide a uniformplating layer. For example, Japanese Patent Application Laid-Open No.62-151592 shows to apply a pulsative current of 1 msec ON time and 25-35msec OFF time to plating electrodes to form a defectless noble metalplating layer. However, since the ON time is very short and no counterpulse is provided during the OFF time, plating layer still containsorganic substances in an amount of 0.05 wt. % -0.08 wt. %.

On the other hand, Japanese Patent Application Laid-Open No. 63-95698shows a solder plating method for obtaining a uniform plated solderlayer on both of conductive circuit pattern and throughhole of amulilayer printed wiring board. Although a counter pulse is applied tothe plating electrodes, there is no mention about the organic substancesin the plating layer. Since the pulse width is selected to be 4.0 msecon less without OFF time and both of the positive and negative pulseshave the same amplitude, it is unknown whether the amount of the organicsubstances in the plating layer is less than 0.03 wt. % or not . Even ifthe amount of the organic substances could be less than 0.03 wt. %, itdoes not anticipate the present invention because the Japanese PatentApplication Laid-Open No. 63-95698 does not mention about the solidelectrolyte capacitor and its solder plated lead terminals connected tothe cathode layer by using a conductive adhesive, much less the problemsof generation of gas and solder ball which is peculiar to themolded-type electronic components having a pre-plated lead terminalspassing through the molded pachage therof.

As described above, the following advantages are realized by plating theanode and cathode lead terminals with a layer of solder or the likeusing the pulse plating method of the present invention.

(1) Because organic substances such as a brightener and/or despersantnormally contained in the plating solution are not incorporated into theplating layer, differing from the case where solder is plated by adirect current plating method, the plating layer formed on the anode andcathode lead terminals has a low organic substance content. Thus, hardlyany gas is generated in the adhering interface between the conductiveadhesive and the cathode lead terminal due to thermal stress whenmouting the finished electronic component. As a result, the connectionreliabilty is enhanced.

(2) In addition, it is possible to eliminate the problem of solder ballgeneration.

It should further be apparent to those skilled in the art that variouschanges in form and detail of the invention as shown and described abovemay be made. It is intended that such changes be included within thespirit and scope of the claims appended hereto.

What is claimed is:
 1. A molded electronic component comprising: anelectronic element having an electrode; a pre-plated lead terminalconnected to said electrode; and an insulating member encapsulating saidelectronic element and leaves a portion of said lead terminals exposed,the encapsulated portions of said pre-plated lead terminals having aplating layer formed thereon containing organic substances in an amountof 0.03 wt. % or less.
 2. The molded electrolnic component as claimed inclaim 1, wherein said electronic component is a solid electnolytecapacitor and said plating layer comprises a solder plating layer. 3.The molded electronic component as claimed in claim 2, wherein an anodelead terminal is welded to an anode lead of said capacitor and a cathodelayer of said capacitor via a conductive adhesive.
 4. The moldedelectronic component as claimed in claim 3, wherein said plating layeris present only on an upper and a lower surface of said anode andcathode lead terminals.